Our invention relates to a nitrodye used for dyeing hair and to a method of dyeing hair using a nitrophenol derivative as the nitrodye.
Nitrodyes currently are widely used in hair coloring agents. They are used in oxidizing hair dyeing compositions as additives for producing natural color shades or artificial color tones which are currently in fashion. By combinations of several different colored nitrodyes it is possible to produce a hair dyeing composition which can dye the hair in natural to artificial shades without use of oxidizing agents.
Thus natural brown colors can be used for example by combination of an orange with a blue nitrodye. Besides it is also possible to combine yellow colored with violet colored nitrodyes to obtain a similar result. Yellow nitrodyes are necessary, which are able to dye the hair either in an intense pure lemon yellow, which must be as free as possible of red component or orange colors and which can be used in combination with blue colors.
There are many other additional requirements however for a hair dyeing composition. The nitrodye must be unquestionable in regard to toxicological and dermatological considerations.
Their use in oxidizing hair dyeing compositions presupposes that they are stable in the presence of hydrogen peroxide in an alkaline solution. Moreover a good light-fastness, acid-fastness and fastness to rubbing is required. Finally the nitrocompounds should be made by the simplest possible process.
The nitrophenol and aminonitrophenols currently described in the literature do not or insufficiently fulfill the previously described prerequisites. Scarcely any hair coloring results from using 2-nitro-phenols. The 4-Nitro-3-(2'-hydroxyethyl)amino-phenol cited in International Journal of Cosmetic Scienc pp. 25 to 35 of course produces a lemon-yellow color but it is however a very weak color. Two additional isomers, namely the nitro and the 5-nitro-2-(2'-hydroxyethyl)amino-phenol, are pH sensitive and show undesirable color changes under action by acid or alkali.
Additional known yellow nitrodyes are the o-, m-, and p-Nitroaniline derivatives taught in German Patent Document 1 619 395. These compounds largely fulfill the requirements regarding dye technology considerations but are not satisfactory in regard to the physiological properties.
The 2-Nitro-aniline derivatives described in German Open Patent Application 3 442 757 largely fulfill the requirements, however the shades of color attainable by variation of the substituents do not extend to the red regions. Thus no fashionable artificial red hair colors can be produced without the additional use of red dyes with the previously described 2-Nitro-aniline derivatives.
Besides it is difficult to obtain a uniform coloring of the hair over the length of the hairs using a hair dyeing compositions having a mixture of dyes, since they are damaged usually at the ends and thus have a more porous surface there than at the hair roots.
A uniform dyeing can be attained, when dyes of the same basic structure are used, which correspond in their selectivity and in their developing ability.
Using dyes of the same structure has the advantage that these dyes also have the same properties in regard to light fastness and washability. Thus for example color differences caused by washing out a dye from a dye mixture by repeated shampooing are avoided.